The present invention relates to a method according to the preamble of claim 1.
There are a large number of different methods, which can be used at detection of presence of gases and measurement of gas concentrations. A common factor for these methods is that they generate a value as a measure of the presence or concentration of a gas. At applications when the detection and measurements will be performed continuously or at a number of successive occasions, it is preferred to use a device, a so-called sensor, which transforms the gas concentration into an electric signal.
The sensors known today include all types of sensors from complex technical systems, like for example mass spectrometers and gas chromatographs, to small and relatively simple sensors, like for example sensors measuring the thermal conductivity of a gas. Most of these sensors measure physical or chemical properties of the atoms or molecules of a gas.
Another type of sensor, which instead measures the presence of molecules of a gas is described in SE-7411342-4. This sensor, which is a semiconductor sensor, exhibits advantages, such as very high sensitivity to and selectivity for hydrogen gas, moderate energy consumption, small size and possibility for rational manufacturing. The term xe2x80x9csemiconductor sensorxe2x80x9d refers herein to the type of sensor described in SE-7411342-4 but includes also other structures of catalytic metals and semiconductors working along the principles described below.
The semiconductor sensor comprises a catalytic metal layer, which captures hydrogen molecules and decomposes these molecules into hydrogen atoms, which diffuse through the metal layer and give rise to an electric signal in the semiconductor structure. The amount of hydrogen atoms within the metal and the amount of hydrogen gas in the surroundings of the metal, equilibrate after a certain time. Thus, the output signal from the semiconductor sensor is dependent of the hydrogen gas concentration in its surroundings. The output signal is also depending on the relationship between the content of oxygen gas and hydrogen gas in the surroundings, which will be described below.
As previously known, the semiconductor sensor provides greater signals for hydrogen gas when the measurements are performed in an environment free of oxygen compared to measurements in an environment containing oxygen. Oxygen in the surroundings of the semiconductor sensor influences the measurements by producing an adsorbed oxygen layer on the metal surface of the semiconductor sensor. The higher the concentration of oxygen gas in the surroundings of the semiconductor sensor, the greater the number of molecules and atoms of oxygen adsorbed on the metal surface. This implies, that the number of sites, which molecules of hydrogen gas can be adsorbed to, is being reduced concurrently with the number of molecules and atoms of oxygen being increased on the metal surface. Furthermore, oxygen reacts with hydrogen adsorbed to the metal surface with a resulting production of water and hydrogen is thereby removed from the metal surface without having influenced the output signal.
Most gas samples subject to analysis regarding hydrogen, contains air and/or oxygen and as it is relatively difficult and complicated to purify gas samples from oxygen in an effective and reproducible way, there has been no practical way to take full advantage of the sensitivity of semiconductor sensors. Purification of gas samples from oxygen also results in that the total analysis time will be considerable lengthened, as this requires an extra step of sample preparation.
Consequently, oxygen is counteracting the sensitivity of the semiconductor sensor for hydrogen gas and influences both the equilibrium signal, i e the output signal when equilibrium between the amount of hydrogen gas in the surroundings and hydrogen within the metal is obtained and the time derivative of the output signal, i e the rate with which the output signal increases at increased hydrogen gas concentration. When interpreting the output signal of the semiconductor sensor, it is previously known to, either use the equilibrium signal or the time derivative of the output signal.
The aforementioned interactions between oxygen and a semiconductor sensor are also valid for carbon monoxide, which also is present in many gas samples.
The object of the present invention is to provide an increased sensitivity for hydrogen gas of the semiconductor sensor. This is achieved according to the method of the invention by means of the measurements indicated in the characterizing part of claim 1.